全文获取类型
收费全文 | 586343篇 |
免费 | 5293篇 |
国内免费 | 2095篇 |
专业分类
化学 | 287602篇 |
晶体学 | 8669篇 |
力学 | 29617篇 |
综合类 | 9篇 |
数学 | 91751篇 |
物理学 | 176083篇 |
出版年
2021年 | 4265篇 |
2020年 | 4586篇 |
2019年 | 5246篇 |
2018年 | 16545篇 |
2017年 | 16493篇 |
2016年 | 15729篇 |
2015年 | 6835篇 |
2014年 | 10010篇 |
2013年 | 24552篇 |
2012年 | 21579篇 |
2011年 | 31569篇 |
2010年 | 20814篇 |
2009年 | 21109篇 |
2008年 | 25763篇 |
2007年 | 27715篇 |
2006年 | 18459篇 |
2005年 | 17445篇 |
2004年 | 16393篇 |
2003年 | 15111篇 |
2002年 | 14148篇 |
2001年 | 15662篇 |
2000年 | 11842篇 |
1999年 | 9223篇 |
1998年 | 7745篇 |
1997年 | 7642篇 |
1996年 | 7219篇 |
1995年 | 6413篇 |
1994年 | 6199篇 |
1993年 | 6222篇 |
1992年 | 6717篇 |
1991年 | 6908篇 |
1990年 | 6610篇 |
1989年 | 6511篇 |
1988年 | 6336篇 |
1987年 | 6365篇 |
1986年 | 6054篇 |
1985年 | 7898篇 |
1984年 | 8068篇 |
1983年 | 6652篇 |
1982年 | 7044篇 |
1981年 | 6707篇 |
1980年 | 6574篇 |
1979年 | 6825篇 |
1978年 | 7062篇 |
1977年 | 6888篇 |
1976年 | 7039篇 |
1975年 | 6455篇 |
1974年 | 6534篇 |
1973年 | 6806篇 |
1972年 | 4591篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
931.
932.
R. A. Gavar L. Kh. Baumane Ya. P. Stradyn' S. A. Giller 《Chemistry of Heterocyclic Compounds》1974,10(3):280-284
Anion radicals of 2-vinylene derivatives of 5-nitrofuran were obtained by electrochemical generation. The hyperfine structure (hfs) of the ESR spectra of these anion radicals indicates delocalization of the unpaired electron over their entire π-electron system. The effect of the vinylene grouping on the distribution of the unpaired electron in the anion radicals and on the distribution of the unpaired electron in the vinylene grouping itself was examined on the basis of the hfs constants. It was found that the vinylene grouping by localizing more than 10% of the density of the unpaired electron on itself reduces, by a factor of 1.4, the effect of substituents in the 2 position of 5-nitrofuran on the distribution of the spin of the unpaired electron in the nitrofuran framework. 相似文献
933.
934.
R. O. Kochkanyan M. M. Nechitailov P. Byszewski A. N. Zaritovskii A. F. Popov 《Russian Journal of Organic Chemistry》2004,40(7):957-961
Reaction of C60 fullerene with ferrocene in the presence of aluminum chloride and aluminum in an inert atmosphere gave ferrocenyl-substituted fullerene 2C60 ·1.5(C5H5)2Fe ·3H2O. The product is a thermally stable iron-containing carbon cage in which the intersphere cavity is formed by three C60 polyhedra and is closed by cyclopentadienyl ligands via formation of four carbon-carbon bonds. 相似文献
935.
U. G. Magomedbekov Kh. M. Gasanova U. G. Gasangadzhieva K. A. Abdulkhamidov S. Sh. Mutsalova 《Moscow University Chemistry Bulletin》2007,62(4):230-236
A possible mechanism of 1,4-naphthalenediol oxidation in the oscillatory regime has been considered, and a mathematical model describing the kinetics has been developed. Based on a thermodynamic Lyapunov function, it has been shown that the source of chemical instabilities is in the existence of autocatalytic steps and dynamic feedbacks. Qualitative analysis and numerical solution of the set of differential equations that model the reaction kinetics have been performed. The character of the stationary state and the possibility of bifurcations have been determined. The mathematical model satisfactorily describes the processes occurring in the system. 相似文献
936.
R. D. Firn 《Photochemistry and photobiology》1990,52(1):255-260
Abstract— The ill defined nature of the conceptual framework used to guide research on phototropism is a handicap to progress. It is argued that some of the central concepts, such as the stimulus perception-transduction-response chain, have not given a useful sense of direction. Likewise the arguments as to whether hormones are involved in phototropism have now become a distraction. A constructive phase is now needed, building on an agreed framework. There is an urgent need to identify the universal elements of phototropism and to distinguish those elements from the ones which apply only in specialised cases. 相似文献
937.
S. S. Mochalov T. P. Surikova Yu. S. Shabarov 《Chemistry of Heterocyclic Compounds》1976,12(7):738-741
5-Bromo- and 5-methoxy-3-ethylanthranils, respectively, were obtained by cyclization of o-nitrosopropiophenone under the influence of hydrogen bromide in benzene and hydrogen chloride in methanol. In these reactions, the starting nitroso ketone undergoes redox transformations that also proceed readily in an inert solvent in the absence of any cyclizing reagents.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 7, pp. 886–890, July. 1976. 相似文献
938.
The dissociation constants of diprotonated 3,3'-dimethylnaphthidine (DMN) and 3,3'-dimethoxybenzidine (DMB) have been determined spectrophotometrically. They are: pK(a1) = 2.62 +/- 0.03, pK(a2) = 3.33 +/- 0.09 for DMN: pK(a1) = 2.83 +/- 0.07, pK(a2) = 4.05 +/- 0.12 for DMB. The molar absorptivities (l.mole(-1).cm(-1)) of all forms of the indicators have been also determined: epsilon(B) = 1.68 x 10(4), epsilon(BH(+)) = 9.34 x 10(3), epsilon(BH(2+)(2)) = 1.80 x 10(3) at 300 nm for DMB; epsilon(B) = 7.33 x 10(3), epsilon(BH(+)) = 3.73 x 10(3), epsilon(BH(2+)(2)) = 0 at 330 nm for DMN. 相似文献
939.
940.
N. S. Prostakov Keti Sudzhi N. M. Mikhailova L. A. Murugova V. F. Zakharov 《Chemistry of Heterocyclic Compounds》1981,17(10):1038-1042
1-Methyl-1H-indeno[1,2-b]pyridine and 1-methyl-1H-5-(, -dicarbomethoxyvinyl)-(formyl, acetyl)indeno[3,2-b]pyridines were obtained by treatment of N-methyl-4-azafluorenium iodide, as well as mixtures of it with acetylenedicarboxylic ester, dimethylformamide (DMF), and phosphorus oxychloride or acetic anhydride, with bases. 4-Azafluoronenone was used to synthesize 9-(p-methoxyphenyl)-4-azafluoren-9-ol, which was reduced to 9-(p-methoxyphenyl)-4-azafluorene, and 1-methyl-1H-5-(p-methoxyphenyl)indeno[3,2-b]pyridine was obtained from the methiodide of the latter.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 10, pp. 1382–1386, October, 1981. 相似文献